1. Field of the Invention
The present invention relates to a magnetic thin film and a method for forming the film, and to a magnetic thin film-applied device, in particular to a magnetic thin film that comprises an alloy having an L11-type ordered structure and to a method for forming the film, and also to a device to which the film is applied. The magnetic thin film-applied device includes typically magnetic recording media such as perpendicular magnetic recording media, as well as tunnel magneto-resistance (TMR) devices, magnetoresistive random access memories (MRAM), microelectromechanical system (MEMS) devices; and in addition, the film is widely applicable to other known magnetic thin film-having devices and the like in accordance with their needs.
2. Background Art
Of those magnetic thin film-having devices, magnetic recording media, tunnel magneto-resistance (TMR) devices, magnetoresistive random access memories (MRAM), MEMS devices and the like are described in brief. First described is a magnetic recording medium.
A magnetic recording system for use in a magnetic recording apparatus, such as a hard disc, a magneto-optical (MO) disc, a magnetic tape or the like includes two types of a longitudinal magnetic recording system and a perpendicular magnetic recording system. For the magnetic recording system for a hard disc, a longitudinal magnetic recording system where magnetic recording is attained longitudinally to the surface of a disc has been employed for many years, but from about 2005, a perpendicular magnetic recording system where magnetic recording is attained perpendicularly to the surface of a disc has become employed as enabling a higher recording density; and as a result, perpendicular magnetic recording media have become used for magnetic recording media. Perpendicular magnetic recording media are disclosed, for example, in Patent Reference 1. Patent Reference 1 discloses examples of perpendicular magnetic recording media excellent in noise reduction, thermal stability and writability, enabling high-density recording and bringing about cost reduction.
FIG. 1 and FIG. 2 are examples of the constitution of the perpendicular magnetic recording medium disclosed in Patent Reference 1. The perpendicular magnetic recording medium shown in FIG. 1 has a structure of an underlayer 3, a magnetic layer 4 and a protective layer 5 formed in that order on a non-magnetic substrate 1. The perpendicular magnetic recording medium shown in FIG. 2 has a structure where a seed layer 2 is provided between the underlayer 3 and the non-magnetic substrate 1 for the purpose of controlling the crystal alignment and the crystal particle size in the underlayer 3.
Also known is a perpendicular double-layer medium having a soft magnetic layer provided between a substrate and a perpendicular magnetic layer. A soft magnetic layer may be provided between the non-magnetic substrate 1 and the underlayer 3 in FIG. 1; and between the non-magnetic substrate 1 and the seed layer 2 in FIG. 2.
As the material for the magnetic recording layer (magnetic layer) of the perpendicular magnetic recording medium, at present, a CoPt-base alloy crystalline film is mainly used, and for use for perpendicular magnetic recording, the crystal alignment in the film is so controlled that the c-axis of the CoPt-base alloy, which has a hexagonal closed packed (hcp) structure, could be perpendicular to the film surface (that is, the c-plane thereof could be parallel to the film surface).
As one system of magnetic layer structure control, proposed is a magnetic layer having a structure of a ferromagnetic crystal grain surrounded by a non-magnetic non-metallic substance such as oxide or nitride, generally referred to as a granular magnetic layer, in a perpendicular magnetic recording medium. It is considered that, in the granular magnetic film of the type, the non-magnetic non-metallic grain boundary phase could act to physically separate the ferromagnetic grains from each other, in which, therefore, the magnetic interaction between the ferromagnetic grains could be lowered and the formation of zigzag magnetic walls to occur in the transition region of recording bits could be inhibited, thereby bringing about noise reduction.
Aiming at further increase in the recording density in perpendicular magnetic recording media, development of discrete track media (DTM) having grooves formed between tracks for reducing the magnetic interferences of the neighboring tracks therebetween, and also development of bit patterned media (BPM) having magnetic grains artificially regularly aligned therein for enabling one bit recording by one magnetic grain are actively under way in the art.
Further, for making it possible to record on a magnetic film having a high coercive force, a heat-assisted magnetic recording (HAMR) or thermal-assisted magnetic recording (TAMR) system is under investigation, and magnetic recording media applicable to the recording system are also under investigation.
Next described are tunnel magneto-resistance (TMR) devices, magnetoresistive random access memories (MRAM), etc. In conventional memories such as flash memories and DRAM, recording is based on the electrons inside the memory cell; however, as opposed to these, MRAM is a memory technique in which a same magnetic substance as in hard discs or the like is used as the memory medium therein. FIG. 3 shows the schematic constitution of MRAM (source: “Current Situation and Problems of Spin-Transfer Magnetization Switching”, “Materia Vol. 42, No. 9”, Sep. 20, 2003, written by Kojiro Yagami et al., issued by the Japan Institute of Metals, p. 646, FIG. 10). In MRAM, the address access time is on a level of 10 nsec and the cycle time is on a level of 20 nsec, which are about 5 times those in DRAM; or that is, MRAM enables rapid reading and writing, as comparable to SRAM. Another advantage of MRAM is that its power consumption is about 1/10 that of flash memory and is low, and it enables large-scale integration.
As in FIG. 3a, “TMR effect” is applied to MRAM, in which a thin insulator film having a thickness of about a few atoms is sandwiched between two magnetic thin films in such a manner that the magnetization direction of one magnetic thin film is varied relative to that of the other to thereby change the resistance value of the system. Specifically, as in FIG. 3b, the tunnel magneto-resistance (TMR) device is used in MRAM.
As another embodiment, the tunnel magneto-resistance (TMR) device may have a structure of a ferromagnetic thin film formed on an antiferromagnetic thin film. The tunnel magneto-resistance device having the structure of the type is disclosed, for example, in FIG. 5 in Patent Reference 9. In addition, FIG. 4 in Patent Reference 9 discloses a spin valve-type magneto-resistance device; and like the above-mentioned tunnel magneto-resistance device, this also has a structure of a ferromagnetic thin film formed on an antiferromagnetic thin film.
Next described is an MEMS device. MEMS is meant to indicate a device where mechanical parts, sensors, actuators and electronic circuits are integrated on one silicon substrate, glass substrate, organic material or the like. Its technical application examples include DMD (digital micromirror device), a type of optical devices in projectors, as well as micronozzles and various sensors such as pressure sensors, acceleration sensors, flow rate sensors and the like as in the head of inkjet printers, etc. In future, its application is expected not only in the filed of manufacturing industry but also in the field of medicine. In the MEMS device, a magnetic thin film is partly used.
In the above-mentioned various devices, it is desired to enhance the magnetic properties of the magnetic thin film, and the development of perpendicular magnetization films having a large uniaxial magnetic anisotropy, Ku is indispensable for further advanced capacity increase and density increase in recording media and memories in future. In particular, in perpendicular magnetic recording media, a magnetic recording layer having a double-layered particle or dot with a hard layer and a soft layer, such as ECC (exchange-coupled composite), hard/soft stacked, exchange-spring or the like layer, has been proposed as the magnetic recording layer in the coming high-density magnetic recording media. However, for fully exhibiting the properties of those media to realize the good thermal stability and the excellent saturation recording properties thereof, it is necessary to use a perpendicular magnetization film having a uniaxial magnetic anisotropy, Ku on a level of 107 erg/cm3, as the hard film in the media. On the other hand, also in spin-transfer magnetization switching MRAM that are expected as high-density memories in future, a perpendicular magnetization film having a large uniaxial magnetic anisotropy, Ku on a level of 107 erg/cm3 is being used for realizing further advanced capacity increase in those memories.
In the above, the Ku unit is expressed by “erg/cm3”; and when it is converted into the corresponding SI unit, then erg/cm3=10−1 J/m3 shall be employed. In case where the unit “emu/cm3” of saturation magnetization to be mentioned below is converted into the corresponding SI unit, then 1 emu/cm3=103 A/m shall be employed.
As the above-mentioned perpendicular magnetization film having a large Ku, Non-Patent Reference 1 discloses a Co—Pt L11-type ordered alloy film. In addition, Non-Patent Reference 2 and Patent Reference 8 disclose an Fe—Pt L10-type ordered alloy film. Further, Patent References 2, 3, 4, 5, 6 and 7 disclose magnetic recording media comprising an L10-type ordered alloy such as an Fe—Pt ordered alloy, an Fe—Pd ordered alloy or a Co—Pt ordered alloy, or a magnetic layer thereof.
In particular, a further increased Ku can be expected by increasing the degree of order of the Co—Pt L11-type ordered alloy film described in Non-Patent Reference 1.
However, for application to perpendicular magnetic recording media, the magnetic film must have not only an increased large uniaxial magnetic anisotropy Ku but also a suitably controlled saturation magnetization Ms. This is described in detail, for example, in Non-Patent Reference 3. The left column on page 180 in Non-Patent Reference 3 says as follows, based on FIG. 3 and FIG. 4: “The saturation magnetization value Ms is thermally stable within a region of from 300 to 700 emu/cm3, and even in no consideration of the reduction in the coercive force owing to thermal decay, the saturation magnetization Ms must be at most 600 emu/cm3 in order to keep the squareness ratio of the magnetization curve at 1.”
Specifically, the magnetic thin film for use in various devices must have a high uniaxial magnetic anisotropy Ku while having a reduced saturation magnetization Ms in accordance with the object thereof.
Patent Reference 1: JP-A 2006-85825
Patent Reference 2: JP-A 2002-208129
Patent Reference 3: JP-A 2003-173511
Patent Reference 4: JP-A 2002-216330
Patent Reference 5: JP-A 2004-311607
Patent Reference 6: JP-A 2001-101645
Patent Reference 7: WO2004/034385
Patent Reference 8: JP-A 2004-311925
Patent Reference 9: JP-A 2005-333106
Non-Patent Reference 1: H. Sato, et al., “Fabrication of L11 type Co—Pt ordered alloy films by sputter deposition”, J. Appl. Phys., 103, 07E114 (2008)
Non-Patent Reference 2: S. Okamoto, et al., “Chemical-order-dependent magnetic anisotropy and exchange stiffness constant of FePt (001) epitaxial films”, Phys. Rev. B, 66, 024413 (2002)
Non-Patent Reference 3: Y. Inaba, et al., “Magnetic Properties of Hard/Soft-Stacked Perpendicular Media Having Very Thin Soft Layers with a High Saturation Magnetization”, J. Magn. Soc. Jpn., 31, 178 (2007)